1. Field of the Invention
The present invention generally relates to a method for fabricating nanowire thermoelectric devices.
2. Description of the Related Art
Nanowires are known in the art as wire structures which have a diameter measured in hundreds of nanometers (nm) or less, typically measured from 1 to 500 nm. When devices are constructed using structures of such a small scale, quantum mechanical rules and phenomena begin to have a greater effect on the operation of such devices in comparison to larger scale structures. The increased role and effect of quantum mechanics is due to the reduced number of atoms and electrons present in the system, which makes their discrete quantum natures more apparent.
While it is not known how quantum mechanics will affect the operation of all nanoscale devices, it has been found that thermoelectric modules (TEMs) constructed using nanowires are likely to show increased efficiency relative to even microsized TEM devices made with bulk materials.
The basic unit of a thermoelectric module is the thermocouple. A thermocouple which is typically made of two thermoelements of different materials, a p-type thermoelement and a n-type thermoelement, connected to each other at a high temperature side and a low temperature side. P-type thermoelements, made from p-type materials, transport charge through holes where electrons are missing. N-type thermoelements, made from n-type materials, transport charge with electrons which travel through the material.
At the high temperature side and the low temperature side, electrodes (p-n connection electrodes) may be provided to connect the two thermoelements. If appropriate materials are chosen for the thermoelements, applying a temperature difference between the two sides develops an electric current at the p-n connection electrodes and, conversely, applying a current to one of the p-n connection electrodes results in a temperature difference between the two sides. Thus, a thermoelectric module may function as either an electric generator or a cooling device.
It is known in the art that the amount of electric current produced by a thermocouple is proportional to the difference in temperature between the two sides of the thermocouple. This phenomena is known as the Seebeck effect, wherein heat energy is converted into electrical energy. Consequently, the Seebeck effect has been identified as a tool for recovering excess or waste heat energy.
The ability of a thermocouple to transfer heat with the application of an electric current is known as the Peltier effect. The amount of heat a thermocouple will transfer depends in part on the magnitude of the current applied. This effect has been employed to provide refrigerators and circuit cooling devices with no mechanical parts.
Another determining factor in the ability of a thermocouple to either absorb heat or produce electricity is the materials used in the p-type and n-type thermoelements. The thermoelectric property of a material is measured in terms of a dimensionless figure of merit (ZT). ZT is defined as follows:ZT=(S2σ/κ)Twhere S is the material's Seebeck coefficient, σ is a measure of the material's electrical conductivity, κ is a measure of the material's thermal conductivity, and T is the temperature.
As is apparent from the definition of ZT, desirable thermoelectric materials have high electrical conductivity, yet low thermal conductivity. Early research in the area of thermoelectrics focused on the use of metals as thermoelements. However, since the relationship between electrical conductivity and thermal conductivity for metals is fixed, the research yielded limited success. Consequently, attention was shifted to develop the use of semiconductors in thermoelectric modules, since semiconductors are capable of both high electrical conductivity and low thermal conductivity.
As noted above, it has been suggested that thermoelectric modules be constructed using nanowires, since some studies have indicated that quantum effects enhance thermoelectric effects. Nanowire thermoelectric modules are envisioned as arrays of nanowires, where p-type and n-type nanowires alternate spatially and are close in proximity to each other so that they may be coupled together. Ideally, all the wires are packed tightly together to increase the number of thermocouples within a thermoelectric module in relation to the size of the module. Significant efforts are now being directed towards the development of such thermoelectric modules.
However, nanoscale elements are not easily produced by conventional methods. In addition, since generation of electricity normally entails a large temperature gradient between the two sides of thermocouples, it is preferred that the thermoelements be as long as possible to increase the distance between the two sides. This is difficult for nanowires and their small diameters because increased length would increase the fragility of the nanowires. Conventional methods typically would not meet tolerances required for the production of nanowire thermoelectric modules and would likely damage their fragile structures.